The optical signal-to-noise ratio (or OSNR for short) is an important quality parameter for recording and/or determining the signal quality and for error diagnosis in optical transmission systems, in particular in long-distance traffic systems using wavelength division multiplexing (or WDM for short). The OSNR is defined as the quotient of the mean signal power and the mean noise power over a defined wavelength interval. Typically, the interval widths that are used are 1 nm or 0.1 nm, corresponding to a frequency interval of 125 GHz or 12.5 GHz, respectively, at 1550 nm.
There are numerous methods for determining the OSNR. The OSNR is normally determined by optical measurements, for example using an optical spectrum analyzer. If the channel separations are very small, for example 25 or 50 GHz, however, the values for the signal power and the noise power are difficult to separate, as a result of which OSNR measurement is virtually impossible during operation. In another optical method, the so-called “polarization nulling” method, the signal is separated on the basis of its defined polarization from the unpolarized noise, by means of polarization filters. However, this method is rather inaccurate, for example because the data signal is partially depolarized as a result of polarization mode dispersion. In addition, the complexity is relatively high because of the additionally required polarization control. In a further optical method, the OSNR is determined by briefly switching off the channel to be measured for a period in the sub-millisecond range, but this is not possible during operation.
Electrical methods also exist for determining the OSNR, in which the OSNR is determined by opto-electrical conversion of a data signal in the receiver. European patent application EP1303062 discloses a method in which the bit error rate (BER) is measured as a function of the decision threshold and is also evaluated, inter alia, with regard to the OSNR. In particular, the method also requires measurements at very high BERs close to 0.5. The areas with a high BER are located at the top and bottom edges in the eye diagram, while the areas with a low BER can be found in the inner and central area of the eye diagram. If the BER values in these areas are extrapolated, this results both in two decision threshold values for the high BERs and in two decision threshold values for the lower BERs. The size of the eye opening can be calculated from the ratio of the differences between these threshold values. If the BER is expressed by the Q-factor as in FIG. 4 of the European application, then the OSNR can be determined by determining the intersection of the two outer straight lines for the low BERs. This method has the disadvantage that it is necessary to know the absolute minimum BER in order to determine the OSNR and, because of the measurements which are required close to BER=0.5, the bit errors which occur during the measurement process can no longer be corrected by an error correction unit FEC (forward error correction).